The structure of a transformer is prescribed by IEC (International Electrotechnical Communication) Standards Pub. 60950, and the like. That is, these standards provide that at least three insulating layers be formed between primary and secondary windings in a winding, in which an enamel film which covers a conductor of a winding be not authorized as an insulating layer (an insulation thin-film material), or that the thickness of an insulating layer be 0.4 mm or more. The standards also provide that the creepage distance between the primary and secondary windings, which varies depending on the applied voltage, be 5 mm or more, that the transformer withstand a voltage of 3,000 V applied between the primary and secondary sides for a minute or more, and the like.
According to such standards, as a currently prevailing transformer has a structure such as one illustrated in a cross-sectional view of FIG. 2. In the structure, an enameled primary winding 4 is wound around a bobbin 2 on a ferrite core 1 in a manner such that insulating barriers 3 for securing the creepage distance are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4, additional insulating barriers 3 for securing the creepage distance are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
Recently, a transformer having a structure which includes neither the insulating barriers 3 nor the insulating tape layer 5, as shown in FIG. 1, has started to be used in place of the transformer having the structure shown in the cross-section of FIG. 2. The transformer shown in FIG. 1 has an advantage over the one having the structure shown in FIG. 2, in being able to be reduced in overall size and dispense with the winding operation for the insulating tape.
In manufacturing the transformer shown in FIG. 1, it is necessary, in consideration of the aforesaid IEC standards, that at least three insulating layers 4b (6b), 4c (6c), and 4d (6d) are formed on the outer peripheral surface on one or both of conductors 4a (6a) of the primary winding 4 and the secondary winding 6 used.
As such a winding, a winding in which an insulating tape is first wound around a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession, so as to form three insulating layers that are separable from one another, is known. Further, a winding in which a conductor is successively extrusion-coated with a fluororesin, in place of an insulating tape, whereby extrusion-coating layers composed of three-layer structure in all are formed for use as insulating layers, is known.
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In the above-mentioned case of extrusion of a fluororesin, since the insulating layer is made of the fluororesin, there is the advantage of good heat resistance. On the other hand, because of the high cost of the resin and the property that when it is pulled at a high shearing speed, the state of the external appearance is deteriorated, it is difficult to increase the production speed, and like the insulating tape, the cost of the electric wire becomes high.
To solve such a problem, a multilayer insulated wire is put to practical use, in which the outer periphery of a conductor is coated, by extrusion, with a modified polyester resin of which the crystallization is controlled, and which is restricted in a reduction in molecular weight, as the first and second insulating layers, and with a polyamide resin as the third insulating layer. Moreover, as a multilayer insulated wire that is more improved in heat resistance, those produced by extrusion-coating with a polyethersulfone resin as the inner layer, and with a polyamide resin as the outermost layer, are proposed.
However, along with recent development of small-sized and high-density electric and electronic machineries and tools, there has been concern about the influence of the heat generated from constituted parts, and the influence of impaired radiating ability. Therefore, higher heat resistance, high chemical resistance, such as resistance to a solvent, from the viewpoint of handling, and also improvements in life time and corona resistance also as to electrical properties, are required. However, insulated wires fulfilling all of these requirements have not been realized at present.